Over the last 20 to 40 years there has been a significant increase in the usage of polymeric materials in the lives o people. Normally this has been as a replacement for other more traditionally used materials because the polymeric material has attributes which lend it to performing the requirements of the task better. One simple and easy example of this is spectacle lenses, where polymeric materials have significantly eroded glass's previous dominance to the case now where more polymeric lenses exist than glass lenses.
Clearly, in order for the polymeric material to provide a true replacement for the traditional material it has to be capable of operating in the true fashion with all forums of additional materials, such as active materials like photochromic materials. With regard to polymeric materials and photochromic materials this is now technically possible, i.e. photochromic materials that are active in polymeric materials are known, and commercially available. However, the addition of the active material to the basic raw polymeric material can drastically increase the price of the material and therefore the final product. An example of this can be illustrated with the materials used for the production of certain types of spectacle lenses:
______________________________________ Cost of basic raw polymer $2.50/lb (for example Polycarbonate) Cost of photochromic loaded Polymer $40.00/lb (for example Polycarbonate) ______________________________________
Now if identical spectacle lenses were made from the respective polymers above clearly approximately the same amounts of polymer would be used. Therefore the spectacle lens made from the photochromic Polycarbonate Polymer is 16 times more expensive to produce than that made from the basic raw polymer. This cost is obviously passed on to the customer.
Now the photochromic material perfectly illustrates the waste of material and increased production costs that can be saved.
With photochromic material in the polymeric material it is active between two different states. A first state or low energy state in which the material is clear, uncoloured and a second state or activated state in which the material is coloured. In order for the material to be activated from the first state to the second state actinic radiation is absorbed. Now with the polymeric materials in which the photochromic material is locked the actinic radiation does not penetrate the lens beyond a depth of above a few millimetres. Consequently only the first few millimetres of the lens will have activated photochromic material, therefore the rest of the photochromic material in the body of the lens is wasted, i.e. not used.